1. Field of the Invention
The present invention relates to a general purpose programmable sequence controller and more particularly, to an output converting circuit for use in such a sequence controller.
2. Description of the Prior Art
Generally, each output converting circuit of a programmable sequence controller is composed of a memory circuit for storing a command signal to energize or deenergize a load, and a load drive element control circuit responsive to the command signal for controlling a load drive element, and these circuits are constructed by semiconductor circuits, such as integrated circuit "IC" logic.
Upon the initial application of power and as the supply voltage rises, the semiconductor circuits are liable to generate their output signals regardless of the logic value of input signals applied thereto, so that the load connected to each output converting circuit may undesirably be erroneously operated.
An output converting circuit used in the past is now described in greater detail with reference to FIGS. 1 and 2. A reference numeral 1 denotes a load drive element comprising relay contact crx, to which a load L is connected. A load drive element control circuit 2 is provided for controlling the opening and closing of the relay contact crx and is composed of a relay coil CRX for the relay contact crx, and inverter IN and a transistor TR. A memory circuit 3 is composed of a flip flop FF for storing a command signal to energize or deenergize the load L. The load L is in connection to an AC 100-volt electric supply 4, while the control circuit 2 is in connection to a DC 5-volt electric supply 5. Onto an output line (a) of the inverter IN is produced a voltage increasing with the supply voltage of the electric supply 5, regardless of the logic level of an input signal applied to the inverter IN upon the initial application of the electric supplies 4 and 5, i.e. the supply voltages thereof, to the associated circuits respectively, and a signal representing the logic of the input signal is output when the supply voltage to the inverter IN is increased to a predetermined voltage level. FIG. 2 (a) shows the wave form of the voltage that appears on the output line (a) of the inverter IN.
The transistor TR, having the output from the inverter IN applied thereto, is also switched on when the supply voltage is increased to a predetermined voltage level. However, the difference in operational characteristics between the transistor TR and the inverter IN causes the same to switch on respectively at different voltage levels, and thus, the collector voltage of the transistor TR may be changed to take such a voltage wave form as indicated in FIG. 2(b). This results in energizing the relay CRX at a certain time as indicated in FIG. 2(c), whereby a brief closing of the relay contact crx is effected to temporarily operate the load L. The temporary operation of the load L, in turn, causes the undesirable operation of an external device, which operation is not based upon an input signal applied to the inverter IN. Particularly, where the external device is used as an interlocking input to another control circuit, an undesireable influence is brought about on the another control circuit.
In order to avoid the foregoing drawbacks, there has heretofore been used an auxiliary relay CRY, as shown in FIG. 4, which is energized when a relay MS1 for hydraulic pump drive is energized for a ready operation by closing a switch PB1 in advance of the starting of control operations, and a normally open contact cry of the relay CRY is connected between the electric supply 4 and a plurality of loads L, as shown in FIG. 3. Further, in the case where no ready operation is performed, there has been used an auxiliary relay CRY, as shown in FIG. 5, which is energized when a time relay TRY energized in response to a start instruction is timed up, and a contact cry of the relay CRY is connected as shown in FIG. 3.
Where in this manner, the auxiliary relay CRY is additionally provided and the relay contact cry is connected serially to the electric supply 4, the relay contact cry is kept necessarily opened at the rise of the supply voltage, and thus, the load L connected to each output converting circuit of the sequence controller can be prevented from being driven even if the load drive element crx of the output converting circuit is temporarily closed. However, in any of the above-noted cases, a malfunction preventing circuit as shown in FIG. 4 or 5 must be provided as an external device in addition to input and output devices of the sequence controller, and this invites a rise of cost due to an increase in number of control circuits components. Further, the provision of the malfunction preventing circuit 4 or 5 not only necessitates the provision of an additional control box and a mounting space therefore, but also brings forth additional works for assembling and wiring.